| The primary objective of this project is to characterize the reflective bands (Bands 1-5 and 7) of Landsat-4 and Landsat-5 Thematic Mapper (TM) sensors using pseudo-invariant sites. Landsat 4 and Landsat 5 take digital images of Earth giving opportunities to study many aspects of our planet and to evaluate the dynamic changes caused by both natural processes and human practices. Thus, it is important to provide accurate radiometric calibration of the reflective bands of TM sensors.;Historically, for purposes of calibration, the Landsat 4 and Landsat 5 TM sensors have used prelaunch calibration data, internal calibration data and vicarious calibration data. Landsat 5, additionally, has used Landsat 7 Enhanced Thematic Mapper plus (ETM+) cross-calibration data. The availability of one more independent source, called pseudo-invariant sites, improves the calibration of the sensor. The pseudo-invariant sites are mainly well characterized desert sites exhibiting the properties of high reflectance, high spatial and spectral uniformity, temporal stability and are nearly Lambertian to decrease BRDF and shadow effects.;The pseudo-invariant sites selected to characterize the Landsat 5 TM sensor are the Libyan site at Path 181, Row 40 and the Altar Site at Path 38, Row 38. Improved vicarious calibration data from the years 2004 and 2005, and the Landsat 7 ETM+ cross-calibration, provided independent absolute calibration sources. The models derived by integrating and comparing all the calibration sources for the sensor indicated a simple exponential model for bands 1-3 and a constant model for bands 4,5 and 7. The models proposed in this paper indicate calibration knowledge of 5% error or better early in life decreasing to nearly 2% later in life.;The pseudo-invariant sites selected to characterize the Landsat 4 TM sensor are the Middle East sites at Path 166, Row 39 and Path 166, Row 40. Previously, calibration attempts have been based only on the internal calibrator and prelaunch data. The Landsat 5 internal calibrator was found to be unreliable approximately three years after launch. The Landsat 4 IC data also become questionable especially knowing that the internal calibrators for both the instruments were made in tandem. Thus, it is necessary to characterize the system with another independent and reliable source---the pseudo-invariant site. Unfortunately, due to the unavailability of adequate number of scenes at the site used for Landsat 5, the Middle East sites of Path 166, Row 39 and Path 166, Row 40 were selected. A large drawback of these sites is that they are the locations of the First Gulf war in 1991. This changed the assumption that the ground data did not change during the study period. To account for the change, radiometric correction was done using the known Landsat 5 gain signature. A correction factor, which is the ratio of the averages of Landsat 5 pre-gulf war and post-gulf war data sets (at-sensor radiance), was used to scale the post-gulf war data of Landsat 4. Both the IC data and pseudo-invariant site data suggested quadratic models for bands 1-4 and linear models for bands 5 and 7. The proposed invariant site models provide consistent calibration knowledge of Landsat 4 TM sensor with differences less than 6% in band 1, nearly 2% in bands 2, 3 and 4 and within 4% in bands 5 and 7, respectively. |
